One of the processes associated with a change in phase is evaporation. This is simply the process of conversion of the liquid phase to vapor phase at an interface. This process occurs whenever there is a concentration difference between the liquid phase and its vapor; e.g., water evaporation into the atmosphere of a room where the relative humidity is less than 100 %.
Boiling is the process in which a liquid evaporates and forms vapor pockets or regions within the continuous liquid phase. Boiling can take many forms. Consider the common everyday occurrence of a pot of boiling water on top of the stove. In this case a stagnant pool of liquid is heated and boiling occurs in the liquid at the bottom of the bulk liquid pool (Figure 1.2). This overall process is called pool boiling. To form the vapor phase within the continuous liquid phase one must heat the liquid to a temperature above its saturation temperature, ( is that temperature at which the liquid exerts a vapor pressure equal to the ambient pressure.) If the temperature of the liquid rises far above (e.g., C for water), the vapor will be formed, "nucleate", as bubbles within the bulk of the liquid causing "volumetric" or "bulk" pool boiling. This type of nucleation is termed "homogeneous nucleation" and is rarely observed in most common boiling situations. Rather, if one were to look at the bottom of the pot of water on the stove one would notice the vapor bubbles nucleating at this heater surface. In this case the temperature of the liquid does not need to be far above Tsat- (e.g., C for water). This nucleation occurs within "preferred-sites: or crevices within the heater surface aided by trapped vapor and gas is termed "heterogeneous nucleation". This type of pool boiling is quite common in many industrial applications.